1. Field of Invention
The present invention pertains to the field of detectors for use in imaging applications including X-ray imaging, fluoroscopy, positron emission tomography (PET), single photon emission computed tomography (SPECT), computed tomography (CT), gamma camera and digital mammography systems. More specifically, the present invention is directed toward the manufacture of a radiation detector using a method for internal manipulation of light waves via the strategic formation of micro-voids, in either the detection media or the light-transmitting media, in a way as to enhance the control and collection of the resultant scintillation light, allowing for the accurate decoding of the impinging radiation.
2. Description of the Related Art
Imaging is widely used in many applications, both medical and non-medical. In the field of imaging, it is well known that imaging devices incorporate a plurality of scintillator arrays for detecting radioactivity from various sources. It is also common practice, when constructing scintillator arrays composed of discrete scintillator elements, to pack the scintillator elements together with a reflective medium interposed between the individual elements creating photon boundaries. Conventionally the reflective medium serves to direct the scintillation light along the scintillator element into a light guide to accurately assess the location at which the radiation impinges upon the detector elements. The reflective medium further serves to increase the light collection efficiency from each scintillator element as well as to minimize the cross-talk, or light transfer (transmission of light), from one scintillator element to an adjacent element. Reflective mediums include reflective powders, films, paints, and adhesives doped with reflective powders, or a combination of materials. Reflective paints and powders contain one or more pigments such as MgO, BaSO4, and TiO2. Regardless of the approach, fabrication of radiation detector arrays is a time- and labor-intensive process, with product uniformity dependent upon the skill level of the workforce. With the current market trend of higher spatially-resolute systems containing an order of magnitude more pixels than current designs, these process effects are even more pronounced.
Detector arrays are commonly integrated with photomultiplier tubes (PMTs) or solid-state detectors such as avalanche photodiodes (APDs), PIN diodes, and charge-coupled devices (CCDs). The incident high-energy photons absorbed by the scintillating material are converted to lower energy scintillation photons, which may be guided to the detectors via one or more of the following: the scintillator itself, a light guide, and other established means of light distribution.
In the arrangement wherein a light guide and/or other established means is used, commonly the light guide is formed by creating slits of various depths in a suitable substrate. Once packed with a reflective media, the light guide becomes an effective method to channel light and to enhance the position information of the scintillator. In the arrangement wherein paint or reflective tape is used, the paint or reflective tape is applied directly to the scintillators, achieving similar results. The height and placement of the applied reflective material varies according to design.
Conventionally, scintillator arrays have been formed from polished or unpolished crystals that are either: hand-wrapped in reflective PTFE tape and bundled together; glued together using a white pigment such as BaSO4 or TiO2 mixed with an epoxy or RTV; or glued to a glass light guide with defined spacing and afterwards filled with reflective material as discussed above.
Another approach utilizes individual reflectors bonded to the sides of certain scintillator elements with the aid of a bonding agent. An array is formed by arranging the individual elements spatially such that the impingement of the high-energy photon is decoded accurately.
Other devices have been produced to form an array of scintillator elements. Typical of the art are those devices disclosed in the following U.S. patents:
U.S. Pat. No.Inventor(s)Issue Date3,936,645A. H. IversonFeb. 3, 19764,749,863M. E. CaseyJun. 7, 19884,914,301Y. AkaiApr. 3, 19904,982,096H. Fujii et al.Jan. 1, 19915,059,800M. K. Cueman et al.Oct. 22, 19915,453,623W. H. Wong et al.Sep. 26, 19956,292,529S. Marcovici et al.Sep. 18, 2001
Of these patents, the '645 patent issued to Iverson discloses a radiation sensitive structure having an array of cells. The cells are formed by cutting narrow slits in a sheet of luminescent material. The slits are filled with a material opaque to either light or radiation or both. The '800 patent issued to Cueman et al., discloses a similar scintillator array wherein wider slots are formed on the bottom of the array.
Most of the aforementioned methods require a separate light guide attached to the bottom of the detector array to channel and direct the light in a definitive pattern on to a transducer or set of transducers such as photomultiplier tubes or diodes. This light guide usually contains slits in varying depths to alter the light pattern onto the transducer(s). In addition the slits are filled with reflective material as discussed in the '863 patent issued to Casey.
The '623 patent issued to Wong et al., teaches a PET camera having an array of scintillation crystals placed adjacent other arrays surrounding a patient area. The edges between the arrays of crystals are offset in relation to the edges between the light detectors, allowing use of circular photomultiplier tubes instead of the more expensive square photomultiplier tubes. This arrangement is referred to as quadrant-sharing, in which each light detector is suitably positioned adjacent four adjacent quadrants of four respective arrays to detect radiation emitted from the four quadrants of each array. The crystals within the arrays are described as being selectively polished and bonded to adjacent crystals to present a cross-coupled interface in order to tunably distribute light to adjacent light detectors. The crystal arrays are formed by optically bonding slabs of crystals into a “pre-array” and then cross-cutting the “pre-array” from one or more sides to form the final array. The grooves may be optically treated, such as with white reflective fillers, for further optical control within the array. In addition, optical jumpers may be coupled to the free end of the array to correct for decoding distortion.
The preparation of light guides and scintillator crystal arrays represent a substantial expenditure in the overall production cost of radiation detectors. Current production means also limit light-channeling geometry to simple rectilinear shapes, due to the increase in complexity of non-rectilinear shapes. An increase in complexity translates into an increase in cost. Additionally, with the current market trend heading towards higher resolution systems containing an order of magnitude more pixels than current designs, cost and labor expenses have become more significant.
An emerging technology that has been used to create ornamental pieces uses laser technology to create three-dimensional images in a transparent material such as glass. Typical of the art are those devices disclosed in the following U.S. patents:
U.S. Pat. No.Inventor(s)Issue Date5,637,244A. I. ErokhinJun. 10, 19975,786,560A. Tatah et al.Jul. 28, 19985,886,318A. V. Vasiliev et al.Mar. 23, 19996,399,914I. TroitskiJun. 4, 20026,417,485I. TroitskiJul. 9, 20026,426,480I. TroitskiJul. 30, 20026,727,460I. TroitskiApr. 27, 2004
Erokhin, in the '244 patent, discloses a method of creating an image inside a transparent material with the aid of a pulsed laser beam. The Erokhin method involves the use of a diffraction-limited Q-switched laser, in particular, a solid-state single-mode TEM00 laser; sharp focusing of the laser beam to provide an adjustable micro-destruction in the material being treated; and mutual displacement of the laser beam and the material being treated after each laser shot to a next point of the image being reproduced. The micro-destruction induced in the material at a pre-set point is adjustable in size by varying the actual aperture of the focusing lens and laser radiation power simultaneously.
The '560 patent teaches a method of treating a material using an ultraviolet (UV) wavelength laser beam having femtosecond pulses. The UV laser beam is split into a plurality of separate laser beams having femtosecond pulses. The separate laser beams are directed onto a target point within a sample such that the femtosecond pulses of the separate beams overlap to create an intensity sufficient to treat the sample.
Vasiliev et al., in their '318 patent, disclose a method for laser-assisted image formation in transparent specimens. The '318 method includes the steps of establishing a laser beam having different angular divergence values in two mutually square planes, and focusing the laser beam at a present point of the specimen. In the course of image formation the specimen is displaced with respect to the point of radiation focusing in order to change an angle between the plane with a maximum laser beam angular divergence and the surface of the image portion being formed so as to suit the required contrast of the image portion involved.
In the patents issued to Troitski, Troitski discloses a system for high-speed production of high quality laser-induced damage images inside transparent materials. The images are produced by the combination of an electro-optical deflector and means for moving the article or focusing the optical system. The Troitski device creates laser-induced damage by generation of breakdowns at several separate centers by using the computing phase hologram, the phase structure of which is calculated so that the laser beam, passing through the hologram, is focused at several spots. The Troitski patent further discloses a system for creation of a laser-induced damage by generation of breakdowns at an area where two laser beams intersect. This decreases the image deterioration conditioned by the use of a deflector and allows to create etch points with different brightness for different directions. In the '914 Troitski patent, one laser is disclosed as generating radiation to heat the material area about a point to the vitrify temperature in order to produce material breakdown.